You can determine when customers see your availability numbers on the public calendar by adjusting the value in:
Admin->System Configuration->Show Availability Threshold.
You can adjust this setting to show your availability: always, never, or only when a certain percentage of seats are still remaining.
Enter a whole number between 0-100 to indicate the percent of available seats remaining.
Customers will only see trip availability displayed on the public calendar after this threshold has been reached.
Eg, if you enter "20" the public calendar will only show the number of seats available for a given trip when 20% or fewer tickets are remaining.
If you enter "100", availability information will always be displayed.
If you enter "0", availability information will never be displayed.